1. Field of the Invention
This invention relates to a chemical vapor deposition apparatus for depositing a chemical vapor film on a workpiece. Still more particularly, the present invention relates to an improved perforated head for a chemical vapor deposition apparatus, which ejects a reaction gas to form the film on a workpiece such as a silicon wafer.
2. Description of the Related Art
Chemical vapor deposition is a process for forming a stable solid by decomposition of chemical vapors using heat, plasma, ultraviolet light or other energy sources. In the following description, a chemical vapor deposition process using heat and a device therefor will be described. This process is widely employed in the fabrication of semiconductor devices, such as metal oxide silicon field effect transistors (MOSFETs). In the production of MOSFETs, a chemical vapor deposit is used to form a silicon epitaxial layer on a silicon wafer. The production yield and the quality of the finished semiconductor device depend on the quality and the uniformity of thickness of the silicon epitaxial layer formed by chemical vapor deposition thereon. It is well known that the hydrodynamical behavior of the relevant reaction gas is critical to the process. In view of this, the characteristics of the ejecting head of the chemical vapor deposition apparatus used for ejecting reaction gas onto the wafer are very important, particularly when the reaction gas flow is perpendicular to the surface of the wafer, as is the case in a low-pressure silicon epitaxial layer forming process. This is because the quality and uniformity of thickness of the silicon epitaxial layer formed thereby are a function of the flow of the reaction gas, which is substantially dependent on the hydrodynamical structure of the ejecting head.
FIGS. 1 and 2 are schematic, cross-sectional elevational views partially illustrating prior art low-pressure chemical vapor deposition apparatuses. In both prior art apparatuses, a bell jar 1 is set on a base plate 15, forming a reaction chamber 17 therewithin. A monocrystalline silicon wafer 3 is placed on a susceptor 4, which is mounted on a graphite heater 2. Reaction gas 7 is ejected downwardly, from a nozzle 5 in FIG. 1 and from a perforated head 6 in FIG. 2, onto the surface of the wafer 3 to form an epitaxial monocrystalline silicon layer on the wafer 3 through heating of the wafer 3 and deposited reaction gas.
The nozzle 5 of FIG. 1 is a simple nozzle pipe which ejects reaction gas 7. The ejected reaction gas 7 directly impinges upon the surface of the wafer 3. Gas molecules in the reaction gas flow arrive at the surface of the wafer 3 at the fastest rate at the center of the flow (and thus the center of the wafer 3 in FIG. 1). The arrival rate of the gas molecules declines toward the periphery of the flow (and the periphery of the wafer 3). The gas flow cools the wafer 3, the most cooling occurring at the center of the flow. This cooling decreases the thermal reaction rate of the reaction gas. As a result, an epitaxial silicon layer 12 formed on the wafer 3 is distributed as illustrated in the cross-sectional elevational view of FIG. 3, with the layer 12 being thickest in a ring immediately surrounding the center of the flow of the reaction gas 7 and decreasing in thickness out to the edges of the wafer 3.
The perforated head 6 of the apparatus of FIG. 2 represents an improvement over the nozzle 5 of FIG. 1 as a reaction gas ejecting means. The perforated head 6 comprises an inverted conical housing 10 having a perforated plate 8 closing the open end thereof. The reaction gas 7 is fed through a pipe 16 which is connected to the housing 10 at the top portion thereof. The perforated plate 8 has numerous small holes 9 therethrough in a matrix pattern The reaction gas 7 is injected from the pipe 16 into the housing 10, flows downwardly, unimpeded, toward the perforated plate 8 and is distributed through the holes 9 from which the gas is ejected onto the surface of the wafer 3.
The deposited silicon layer 13 on the wafer 3 has a satisfactorily uniform thickness, as illustrated in the cross-sectional elevational view of FIG. 4, but has a whitish, dull surface portion 18 in the vicinity of the center of the wafer 3. The generation of the dull surface is caused by the hydrodynamical behavior of the reaction gas 7 in the housing 10.
Molecules of the reaction gas 7 which are injected by the pipe 16 into the housing 10 pass through the holes 9 in the perforated plate 8, forming an ejection flow 11 beneath the perforated plate 8. However, a large quantity of the molecules injected by the pipe 16 impinge on the surface of the perforated plate 8, rather than passing directly through one of the holes 9 on their initial journey downward from the pipe 16. This impinging action is generally nearly perpendicular, and the gas molecules are partially reflected back by the surface of the plate 8. Especially when combined with the continuing downward flow of the reaction gas 7 from the pipe 16, a turbulent flow forms within the housing 10. The turbulent flow of the reaction gas tends to cause the premature formation of silicon particles 14 in the housing 10 rather than as a part of a monocrystalline silicon layer on the surface of the wafer 3. The silicon particles 14 are generated by collisions between the reaction gas molecules in the housing 10. These collisions occur easily when the gas flow is turbulent, even though the gas molecules have fairly long mean free paths at low reaction pressures. The prematurely generated silicon particles 14 eventually pass through the holes 9, join the flow 11 (the highest concentration being near the center of the flow 11) and reach the surface of the wafer 3.
As a result, the central surface 18 of the deposited epitaxial silicon layer 13 corresponding to the center portion of the perforated head 8 and the gas flow 11 has a rather rough surface and a whitish, dull appearance. The central portion 18 of the epitaxial silicon layer 13, moreover, has a polycrystalline structure which is grown due to the deposited silicon particles 14, which act as crystal growth nuclei. Meanwhile, the peripheral region of the deposited layer 13 has a mirror surface, which implies the desired mono-crystalline silicon structure. Achievement of the monocrystalline structure is ascribed to the slow flow rate of the reaction gas 7 in the region of the housing 10 above the peripheral region of the perforated plate 8. No turbulent back flow occurs in the peripheral region and as a result the ejection flow 11 to the surface of the wafer 3 above its corresponding peripheral region is even and contains no prematurely formed silicon molecules.
Therefore, it is clear that a uniformly distributed silicon reaction gas flow to the surface of the wafer 3 containing no silicon particles is desired. A perforated head for achieving such reaction gas flow, and more specifically a perforated head which generates thereinside a laminated gas flow for ejection onto a wafer, is clearly needed.